Motors are, in general, subject to eccentricities due to run-out of their shafts. This eccentricity must be prevented as much as possible in spindle motors which drive a variety of discs. Since a rotating shaft of a motor, from which shaft an eccentricity is detected, is shaped like a cylinder, a measuring instrument such as a contact-type dial gauge, an electric micro-meter, or a non-contact type electrostatic capacitance displacement gauge, or a laser displacement gauge is generally used for measuring an eccentricity of the rotating shaft.
A specific method of measuring the eccentricity is disclosed in, e.g. Japanese Patent Non-examined Publication No. H05-227710. Besides a first peak sensor that senses a peak value of an output from a displacement gauge, a second peak sensor is provided, so that a difference between two peak values sensed by these two sensors identifies an eccentricity.
In a case of motors driving a polygon mirror, a section subjected to detection is a mirror formed of facets, namely, mirror shapes like a non-cylinder. In this case, measuring an eccentricity with a measuring instrument contacting a subject thing is not allowed, so that a totally different measuring method is required. In this sense, a contact-type sensor cannot be used as a matter of fact. Use of a non-contact type capacitance displacement gauge requires placing the gauge near to the motor as close as several tens μm, so that sensors thereof are subject to collision with an angular polygon mirror. As a result, it is difficult to measure an eccentricity with the non-contact type capacitance gauge. Use of a laser displacement gauge available on the market has a speed of response on the order of micro-seconds, so that a measurable range can be at most several hundreds rotations per minute. Since the polygon mirror rotates several tens of thousand rotations per minute, it is impossible for the laser displacement gauge to measure eccentricity.
Polygon mirror motors are used in laser-beam printers and full-color copiers, and directly influence printing quality, so that a motor of high accuracy is needed. A polygon mirror is directly connected to an output shaft of a brush-less DC motor and is spun at a high speed such as several tens of thousand rotations per minute. A method of measuring a dynamic eccentricity of a motor spinning at such a high speed is disclosed in, e.g. Japanese Patent Non-examined Publication No. H02-204713.
FIG. 6 shows a structure of a conventional measuring instrument of a polygon mirror motor. Polygon mirror 81 rigidly mounted to rotating shaft 80 of the motor spins at a high speed. Laser beam L1 emitted from first laser light beam source 82 enters into polygon mirror 81 at a certain angle, and reflected laser beam L2 passes through cylindrical lens 87 and travels to position detector 83 which detects a position of the laser beam having undergone lens 87.
On the other hand, laser beam L3 emitted from second laser light beam source 84 passes through half mirror 85, and passed laser beam L4 enters to polygon mirror 81. A reflected laser beam L5 enters half mirror 85, and its reflected light beam L6 enters to trigger generator 86.
As shown in FIG. 6, when a reflective surface of polygon mirror 81 and incident laser beam L4 form a right angle, reflection beam L5 enters to half mirror 85, and its reflection light beam L6 enters to trigger generator 86. Thus when polygon mirror 81 becomes a status as shown in FIG. 6, trigger generator 86 generates trigger signals, and observation of output signals from position detector 83 at this time allows measuring an eccentricity and a level difference between mirror facets.
Position detector 83 employs a one-dimensional position sensitive detector (PSD) which is best suited to this kind of measuring because of its following features: PSD is a position sensor of a spot light and makes use of a surface resistance of a photo-diode, and it can obtain consecutive analog signals and is excellent in terms of responsiveness. However, since this position detector outputs analog signals, an output therefrom changes over time or due to environmental changes, and a change of the output causes an error in measuring. This detector is expensive, so that it increases cost of the measuring instrument.